This invention relates to coated phosphor particles, and more particularly relates to such particles having vapor-deposited coatings, and also relates to a method for producing such coated particles, and to cathode ray tube screens incorporating such particles.
Recent interest in color shifting of cathode ray phosphors, is directed toward increasing the visual contrast of the image on a television cathode ray tube screen. Such color shifting of T.V. phosphors is generally accomplished by the addition or adjustment of dopant materials in the phosphor material, or by coating the individual phosphor particles with a pigment, such as by precipitation from a solution or slurry. Such coatings accomplish the desired color shift by selectively absorbing or filtering a portion of the light output of the phosphor particles. Of course, a corresponding loss of brightness is traded for the achievement of the desired color shift. An added benefit is also sometimes realized in that the pigment changes the body color of the phosphor particles from a light reflective color to a more absorptive (darker) color, thereby further enhancing visual contrast of the screen, especially in the presence of significant ambient light.
Typically in a tri-color cathode ray tube for color television having dots or stripes of red, blue and green phosphors on the cathode ray tube screen, the red and blue phosphors are pigmented. Typical red pigment materials are cadmium sulfo-selenide, red iron oxide and certain rare earth phosphates. A typical blue pigment is cobalt aluminate.
One method of producing such pigmented particles is described in U.S. Pat. No. 3,275,466, wherein the phosphor particles are immersed in a solution of gelatin to coat the particles. The gelatin-coated particles are then agitated in a suspension of pigment particles which are one to two orders of magnitude smaller in size than the phosphor particles. The filter particles cling to the phosphor particles, giving a partial coverage of the surface.
Another method of producing such coated particles is described in U.S. Pat. No. 3,875,449, wherein there is formed a water suspension of the pigment and phosphor particles, followed by mixing, filtering, drying and heat treating to sinter. The sintering is said to form a monolithic layer of pigment covering the entire phosphor particle.
Another method of producing such coated particles is described in U.S. Pat. Nos. 4,020,231, and 4,128,674, wherein the addition of a base or salt to an aqueous mixture of phosphor particles, pigment particles and colloidal silica or pyrophosphate results in gelling of the mixture and co-precipitation of the pigment and colloidal particles on the phosphor particles.
In U.S. Pat. No. 3,886,394, phosphor particles are covered with a gelatin layer, for example, polyvinyl pyrrolidone, having particles of pigment and silica dispersed therein. In U.S. Pat. No. 4,021,588, a filter material is precipitated directly onto the phosphor particles, and the particles are then sintered to adhere the filter particles to the surfaces thereof. Finally, in U.S. Pat. No. 4,049,845, filter particles are adhered to the surfaces of phosphor particles by means of coagulating colloidal particles of latex from a suspension surrounding the particles therein.
The above coating techniques in general result in coatings which are of uneven thickness or discontinuous, or both. Such techniques typically also rely upon a binder material to adhere the pigment particles to the surface, which binder has light absorptive properties of its own, thus further reducing light output.
In the '449 patent, a monolithic coating of even thickness is said to be achieved by sintering. However, such sintering must be carried out under closely controlled conditions, to avoid sintering together of the phosphor particles into a mass, which must then be reduced again to particulate form, such as by crushing, milling and screening. Such operations could damage the pigment coatings.